The physical properties of organic-inorganic hybrid materials depend appreciably on the strength but also the structure and properties of the interface between the components present therein. Coupling reagents are usually used to amplify the interaction between the polymeric phase and the filler phase. They are frequently functionalized alkyl-, alkoxy- or halosilanes capable of establishing a covalent bond to the filler surface in the course of vulcanization and/or curing of the polymeric system. This conventional process has the disadvantage that often merely a low coupling density is achieved. It also offers only limited scope, if any, for controlled manipulation of the interphase between the polymeric matrix and the filler surface.
Reversible addition fragmentation chain transfer (RAFT) is a promising method of conducting free-radical polymerizations in a controlled manner. The choice and chemical construction of so-called RAFT agents, which contain an R-group and a Z-group, can be varied to control the structural binding of the polymer to the surface. The functional sites of the RAFT agent typically remain directly on the surface during the polymerization (Z-group approach, see FIG. 1 and FIG. 2). The so-called R-groups consequently remain on the polymer and can lead to specific functionalization, albeit structurally limited by the use of the RAFT agent. Specifically, FIG. 2 depicts an immobilized RAFT agent provided on the surface of a filler 100. RAFT polymerization reaction 200 results in the surface modification of the filler with surface-bound polymer 210. The separation and recovery of the RAFT agent in step 220 results in a polymer of well-defined architecture 230 and the immobilized raft agent on the surface of filler 100.
Binding to the filler surface typically proceeds via coupleable silyl substituents, often referred to as Z-group (such as —Si(OAlk)3, —SiAlk(OAlk)2 or —SiAlk2(OAlk) for example), on the particular RAFT agents. Preparing these silane-substituted RAFT agents and the possibility of crosslinking the individual silane groups of silane-terminated polymer chains is described in United States patent application publication 2010/0222504. However, a person skilled in the art already knew of the preparation of silane-substituted RAFT agents, for example from D. H. Nguyen, P. Vana, Polymers for Advances Technologies, 2006, 17, 625 or from L. Barner, T. P. Davis, M. H. Stenzel, C. Barner-Kowollik, Macromol. Rapid Commun., 2007, 28, 539.
Similarly, the attachment of RAFT agents via the coupleable silyl group is described in various publications, for example in C. H. Liu, C. Y. Pan, Polymer, 2007, 48, 3679 or Y. L. Zhao, S. Perrier, Macromolecules, 2006, 39, 8603. Conventional processes comprise the endowment of polymers with end groups that are reactive toward the filler surface and thus enable covalent or other bonding to the filler surface in the course of the mixing or vulcanizing operation. U.S. Pat. No. 6,998,452, for example, discloses a RAFT agent having a Z-group endowed with a coupleable silyl substituent. The Z-group is the functionality which stabilizes the free-radical intermediate following addition of a polymer chain. This method of polymerization is referred to as the “grafting to” approach. Disadvantages here are the low grafting densities and a certain degree of instability, since the thiocarbonylthio group of the control agent is involved in the bonding to the filler. Moreover, localizing the graft group on the filler surface means that further modification of the second, free end of the polymer is no longer possible. What is more, steric hindrances between the propagating chains and the surface-attached chains become increasingly influential in this approach, since the reactive chains first have to diffuse through the surficially bound layer of polymer in order that they may react with the functional RAFT group.
The possibility of RAFT polymerizing particles of carbon black is already known from Q. Yang, L. Wang, W. D. Xiang, J. F. Zhou, Q. H. Tan, Polymer 2007, 48, 3444. However, these authors use a different mechanism of attaching, for which the carbon black needs to be profoundly surface-modified, which is costly and inconvenient in that multiple steps are needed and only minor quantities can be prepared.